Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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High Power Combiner Apparatus
Field of the Invention
The present invention relates to power combiner networks and, more
particularly, to the selection of multiple power levels using power combiners.
Background of the Invention
Power combiners are well-known devices that couple electromagnetic energy
from multiple input ports to an output port in a prescribed manner. As is well-
1o known, high power combiners are used in a number of application such as (i)
combining two or more signals at the same or different frequencies for
transmission
by a common antenna; (ii) combining an analog signal and a digital signal for
common antenna transmission, e.g., digital television and/or digital audio
broadcast
applications; and (iii) combining outputs of multiple power amplifiers.
t 5 The art is replete with power combiner arrangements for use, inter alia,
in
the above-described applications. For example, U.S. Patent No. 4,315,222
issued to
A. Saleh on February 8, 1982, which is hereby incorporated by reference for
all
purposes, describes a power combiner arrangement for microwave power
amplifiers
which employs a series of sensing devices at the inputs to the combiner for
20 identifying failed amplifiers at the inputs thereby improving the
degradation
performance of the microwave power amplifier. U.S. Patent No. 4,697,160 issued
to R. T. Clark on September 29, 1987, which is hereby incorporated by
reference for
all purposes, describes a hybrid power combiner and controller for achieving
power
combination with improved finer amplitude control having reduced insertion
loss.
25 Further, U.S. Patent No. 5,222,246 issued to H. J. Wolkstein on June 22,
1993,
which is hereby incorporated by reference for all purposes, describes a power
amplifier arrangement employing a phase-sensitive power combiner for dividing
a
input signal into equal amplitude components for amplification purposes. As
will be
appreciated, the performance specifications of such power combiners continue
to
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become more varied and stringent with the advent of new and/or expanded
applications.
For example, in the United States AM/FM radio broadcast market, digital
audio broadcast ("DAB") technology, e.g., so-called In-Band On-Channel
("IBOC"), is under consideration for widespread application. Digital audio
broadcast applications are described, e.g., in Carl-Erik Sundberg, "Digital
Audio
Broadcasting in the FM Band", Proceedings of the IEEE Symposium on Industrial
Electronics, Portugal, June 1-1 l, 1997, and Carl-Erik Sundberg, "Digital
Audio
Broadcasting: An Overview of Some Recent Activities in the U.S.", Proceedings
of
1o Norsig-97, Norwegian Signal Processing Symposium, Tromso, Norway, May 23-
24,
1997, each of which are hereby are incorporated by reference for all purposes.
Further, IBOC is described, e.g., in Carl-Erik Sundberg et al., "Technology
Advances Enabling In-Band-On-Channel DSB Systems", Proceedings of Broadcast
Asia, June 1998, Suren Pai, "In-Band-On-Channel: The Choice of U.S.
Broadcasters", Proceedings of Broadcast Asia, June 1998, and B. W. Kroeger et
al.,
"Improved IBOC DAB Technology for AM and FM Broadcasting", SBE
Engineering Conference, pp. 1-10, 1996, each of which are hereby are
incorporated
by reference for all purposes. IBOC broadcasting systems utilize a digital
overlay in
the current FM analog broadcast band to deliver digital audio content. In
accordance with IBOC, lower power digital signals, e.g., 20 to 30 dB below the
analog signal level, are embedded as two sidebands on either side of the
analog
signal transmission within ~ 200 kHz (off center frequency) as is required by
current
FCC regulations. As such, the digital sidebands are immediately adjacent to
the
analog band with virtually no significant separation between the frequencies
of the
analog and digital signals. Therefore, in order to achieve a degree of
compatibility
between the analog and digital signals, a sufficient isolation between the
analog
signal transmitter and digital signal transmitter must be achieved. In
particular, a
higher isolation is required from the analog transmitter to digital
transmitter than
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from the digital transmitter to the analog transmitter because of the
relatively large
differential (e.g., 20 to 25 dB) in power levels between the two signals.
The challenge of achieving higher isolation, e.g., 60 to 80 dB, in an
application such as IBOC, i.e., isolation between power sources where at least
one
source is much higher than the other, is to provide the requisite isolation
with
minimal degradation in insertion loss and group delay variation. As will be
appreciated, depending upon the specific application the term "high power"
will
have different meanings. For example, in cellular applications, high power
typically
means 100W or greater. Further, as will be appreciated, frequency proximity
1o requirements also vary by application and impact such high power
applications.
More particularly, problems arise in high power combining when high isolation
is
required for signals having overlapping or nearly overlapping spectral
occupancy
characteristics. In cases where the signals are spectrally proximate but not
overlapping, prior art high power combiners typically employ filtering in
15 combination with power combining to increase isolation. However, the need
for
severe filter transitions, in the most proximal cases, often leads to
undesirable
distortions of the signals as they undergo the combining process. Furthermore,
those signals to be combined that have overlapping spectral occupancies cannot
benefit from these filtering schemes to increase isolation, but must rely
solely upon
2o inherent isolation of the core combiner.
Therefore, a need exists for a high power combiner with improved isolation
between input ports for high power applications with minimal degradation in
signal
characteristics, e.g., insertion loss and/or group delay variation.
25 Summary of the Invention
The present invention is directed to a high power combiner arrangement with
improved isolation between input ports for high power applications. In
particular, in
accordance with the preferred embodiment of the invention, power combining
logic
is combined with a series of isolators such that at least one isolator is
inserted
3o between at least one power source, i.e., a signal source, and a
corresponding input
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port to the power combining logic. The number and location of isolators
inserted is
determined as a function of the isolation requirements of the overall
application. In
accordance with the preferred embodiment, at least one isolator is a three
port
junction circulator device formed by a symmetrical junction transmission line
coupled to a magnetically-biased ferrite material. Further, in accordance with
preferred embodiments of the invention, the at least one circulator has at
least one
port terminated with a resistive matched load such that when one of the three
ports
of the circulator is terminated with the matched load, the circulator becomes
an
isolator which will isolate the incident and reflected signals at the
remaining two
1 o ports.
Advantageously, in accordance with the invention, the degree of isolation
achieved by the high power combiner is directly proportional to the number of
isolators placed between each power source. Furthermore, the insertion of a
number
of high power circulators between each power source and the power combing
logic
15 facilitates the achievement of higher isolation between the power sources
with
limited degradation in signal characteristics.
In accordance with a further embodiment of the invention, the power
combining logic is a hybrid coupler combined with a series of circulators such
that
at least one circulator is inserted between a power source and a corresponding
input
2o port to the hybrid coupler. As above, the number of circulators inserted is
determined as a function of the isolation requirements of the overall
application.
Brief Description of the Drawings
FIG. 1 shows an illustrative prior art power combiner;
25 FIG. 2 shows an illustrative power combiner configured in accordance with
the preferred embodiment of the invention;
FIG. 3 shows illustrative graphical results of total isolation results
achieved
using the power combiner arrangement of the invention as shown in FIG. 2; and
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FIG. 4 shows an illustrative hybrid power combiner configured in
accordance with a further embodiment of the invention.
Throughout this disclosure, unless otherwise noted, like elements, blocks,
components or sections in the figures are denoted by the same reference
designations.
Detailed Description
The present invention is directed to a high power combiner arrangement with
improved isolation between input ports for high power applications. In
particular, in
t o accordance with the preferred embodiment of the invention, power combining
logic
is combined with a series of isolators such that at least one isolator is
inserted
between at least one power source, i.e., a signal source, and a corresponding
input
port to the power combining logic. The number of isolators inserted is
determined
as a function of the isolation requirements of the overall application. In
accordance
~ 5 with the preferred embodiment, at least one isolator is a three port
junction
circulator device formed by a symmetrical junction transmission line coupled
to a
magnetically-biased fernte material. Advantageously, in accordance with the
invention, the degree of isolation achieved by the high power combiner is
directly
proportional to the number of inserted isolators placed between a power source
and
2o the corresponding input port. Furthermore, the insertion of a number of
high power
circulators between the power sources and the power combing logic facilitates
the
achievement of higher isolation between the power sources with minimal
degradation in signal characteristics.
It should be noted that for clarity of explanation, the illustrative
25 embodiments described herein are presented as comprising individual
functional
blocks or combinations of functional blocks. The functions these blocks
represent
may be provided through the use of either shared or dedicated hardware,
including,
but not limited to, hardware capable of executing software. Illustrative
embodiments may comprise digital signal processor ("DSP") hardware and/or
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software performing the operations discussed below. Further, in the claims
hereof
any element expressed as a means for performing a specified function is
intended to
encompass any way of performing that function, including, for example, a) a
combination of circuit elements which performs that function; or b) software
in any
form (including, therefore, firmware, object code, microcode or the like)
combined
with appropriate circuitry for executing that software to perform the
function. The
invention defined by such claims resides in the fact that the functionalities
provided
by the various recited means are combined and brought together in the manner
which the claims call for. Applicants thus regard any means which can provide
1 o those functionalities as equivalent as those shown herein.
In order to provide context and facilitate an understanding of the invention,
a
brief overview of an illustrative prior art power combiner will now be
discussed.
More particularly, FIG. 1 shows illustrative prior art power combiner 100 as a
well-
known multiport device which couples electromagnetic energy from the incident
to
t 5 the output ports in a prescribed manner. In particular, hybrid coupler 110
is a device
having four ports, ports 140-170, respectively. The ports of hybrid coupler
110 are
configured as follows: power source 120, i.e. a first signal source, is
connected to
port 170, power source 130, i.e., a second signal source, is connected to port
150,
antenna 190 is connected to port 140, and balancing load 180 is connected to
port
20 160. As will be appreciated, part of the signal from power source 120 at
port 170
leaks, in a well-known manner, to port 150 and port 160, respectively, and
part of
the signal from power source 130 at port 150 leaks to port 160 and port 170,
respectively. Further, leakages at port 160 are dissipated in balancing load
180.
As will be understood, one goal in any power combining arrangement, such
25 as power combiner 100, is that signal leakages to any port except the main
output
port, e.g., port 140 of hybrid coupler 110, be minimized to prevent
interference
between the sources. As such, the level of leakage between port 150 and port
170 is
defined as the isolation between these two ports, respectively. For
conventional
commercially available hybrid coupler arrangements, e.g., hybrid coupler 110,
this
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isolation value is typically in the range of 15 to 35 dB. Combining multiple
power
sources requires these signals to be coupled with appropriate phase and
amplitude
relationships which, as is well-known, are achieved in hybrid coupler 110 by
requiring good matches at all ports under all signal conditions. Nevertheless,
the
isolation from one power source to another power source achieved by power
combiner 100 is a direct relation to that which is provided as a function of
hybrid
coupler 110, i.e., an isolation of 20 to 35 dB.
Traditionally, to apply power combiner 100 in high power combining
applications (e.g., in a IBOC DAB application high power ranges from 100W to
to 100kW), the use of filter networks, e.g., bandpass, bandstop, low pass
and/or high
pass filters, have been used to achieve additional isolation between multiple
power
sources, e.g., power source 120 and 130, respectively. Such filter networks
are
inserted, illustratively, in power combiner 100 at either port 170 or port 150
after
power source 120 or power source 130, respectively, in a well-known manner.
15 However, such conventional configurations of power combiners suffer from
certain
drawbacks such as incurring undue insertion losses and/or group delay
variations
when the signals to be combined are close in frequency.
In contrast, we have recognized a high power combiner arrangement with
significantly improved isolation between input ports for high power
applications. In
2o particular, in accordance with the preferred embodiment of the invention,
power
combining logic is combined with a series of isolators such that at least one
isolator
is inserted between a power source and a corresponding input port to the power
combining logic. The number of isolators inserted is determined as a function
of the
isolation requirements of the particular application. In accordance with the
25 preferred embodiment, at least one isolator is a three port junction
circulator device
formed by a symmetrical junction transmission line coupled to a magnetically-
biased ferrite material. Advantageously, in accordance with the invention, the
degree of isolation achieved by the high power combiner is directly
proportional to
the number of inserted isolators placed between the power source and the
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corresponding input port. Furthermore, the insertion of a number of high power
isolators between the power source and the power combing logic facilitates the
achievement of higher isolation between the power sources with minimal
degradation in signal characteristics.
More particularly, FIG. 2 shows illustrative power combiner 200 configured
in accordance with the preferred embodiment of the invention. Power combiner
200
includes power combining network 205, and ports 225-235, respectively, which
provide connections, inter alia, to first power source 210, second power
source 215,
and antenna 220. As such, power combiner 200 is used to effectively combine
the
1 o two signals from power sources 210 and 21 S, respectively, for output
through port
235 to antenna 220. For example, using power combiner 200 the two signals from
power sources 210 and 215 may be signals at the same or different frequencies
which are transmitted by the same antenna, i.e., antenna 220. Further,
illustratively,
using power combiner 200 the two signals from power sources 210 and 215 may be
of different signal types. For example, the signals from the power sources may
be
any combination of analog signals and digital signals which are to be
transmitted
over a common antenna, i.e., antenna 220, such as in a digital television or
digital
audio broadcast applications.
For example, in a IBOC application there is little or no separation between
2o frequencies of the analog and digital signals of such applications. Thus,
to transmit
both the analog and digital signals over the same antenna in an IBOC system,
with
minimal signal degradation, isolation between these signals must suppress
interactions between source signals to ensure that the combined signal will
satisfy
and comply with the predetermined requirements as specified in the so-called
FCC
mask. As will be appreciated, such isolation requirements are primarily a
function
of the class of transmitter station deployed in the digital audio broadcast
system.
Advantageously, in accordance with the invention, the degree of isolation
achieved
by the high power combiner is directly proportional to the number of inserted
isolators placed between each power source. Furthermore, the insertion of a
number
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of high power circulators between each power source and the power combing
logic
facilitates the achievement of higher isolation between the power sources with
limited degradation in signal characteristics.
More particularly, in accordance with the invention, isolators are employed
in the power combiner arrangement to improve the impedance matches at ports
225-
235. In particular, FIG. 2 illustratively shows a series of isolators N,
through N~,
see, e.g., isolator 240 through isolator 245, respectively, displaced between
power
source 210 and port 225 of power combining network 205. As will be
appreciated,
power combining network 205, in accordance with various embodiments of the
1 o invention, can be a hybrid coupler, a so-called Wilkinson
divider/combiner, or
similar combiner circuitry consisting of lumped or distributed components
(e.g.,
resistors, capacitors, inductors, and the like), taken either individually, or
in any
combination, with a filter network at the particular input ports of the power
combing
network 205. Further, power combiner 200 further illustratively shows a series
of
~ 5 isolators M, through Mk, see, e.g., isolator 250 through isolator 255,
respectively,
displaced between power source 215 and port 230. In accordance with the
preferred
embodiment of the invention, isolators 240-260 are shown as well-known
circulator
devices in power combiner 200. As will be appreciated, circulators are
typically
used for directing signals to a particular load using its signal duplexing
device
2o characteristics. Further, isolators are used for the isolation of incident
and reflected
signals in electronic devices. As such, we have recognized that such
circulator
devices can be used effectively in accordance with the principles of the
invention to
deliver a power combiner with significantly enhanced isolation between input
ports
in high power applications with minimal degradation of signal characteristics
as
25 further discussed below.
In addition, in accordance with the preferred embodiment, isolator 260 is
inserted between antenna 220 and the final output, i.e., port 235, of power
combining network 205 to ensure that power combiner 200 is matched with a
sufficient impedance value despite being subject to potentially poor antenna
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impedances resulting, in a well-known fashion, from conditions such as
temperature, frequency and aging. That is, the use of isolator 260 between
port 235
of power combining network 205 and antenna 220 provides a robust interface to
antenna 220 and minimizes RF power reflected from antenna 220 from being
dissipated in power combiner 200 and/or power sources 210 and 215,
respectively.
In addition, by providing robust termination impedance the optimal isolation
performance of combiner 200 is optimized.
More particularly, isolators 240-260, are each a three port junction
circulator
device formed by a symmetrical "Y" junction transmission line coupled to a
1 o magnetically-biased ferrite material. As will be appreciated, the
combination of the
ferrite material, magnetic bias and transmission line realization determines
the
actual power handling capability of the circulator. That is, when one of the
three
ports of the circulator (see, e.g., circulator 240 having ports 201, 202, and
203,
respectively) is terminated with a matched load, the circulator becomes an
isolator
which will isolate the incident and reflected signals at the remaining two
ports. For
example, with respect to circulator 240, a signal incident at port 201 is
directed to
port 202 of circulator 240. If there is a matched load, e.g., matched load
280, a large
percentage of the power proportional to the so-called return loss of the load
at port
202 is dissipated in matched load 280 at port 202. When the load at port 202
is very
2o well matched, e.g., with a return loss of -20 dB or better, only a
particular ratio of
the power incident at port 202 will be reflected or directed to port 203 and
dissipated
in the matched load at port 203.
Thus, in accordance with the preferred embodiment of the invention, power
combiner 200 includes matched loads 265-285, with each respective load being
matched to a particular isolator. A typical matched load is a one port device
with a
purely resistive 50 Ohm impedance capable of absorbing incident
electromagnetic
energy and converting such energy to heat for dissipation. For example,
isolator
240 is matched with matched load 275, and isolator 250 is matched with matched
load 265. In accordance with the invention, the number of isolators, e.g.,
circulators,
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placed between a particular power source and corresponding input port is a
function
of the isolation requirements of the application itself. Furthermore, the
typical
isolation realized per circulator, as in the configuration of FIG. 2., is
approximately
20 dB with an incurred insertion loss of less than 1 dB. That is, the higher
the
isolation requirements of the application there is an expected increase in
insertion
loss. Thus, in accordance with the preferred embodiment of the invention, the
selection of the number of isolators in terms of the isolation requirements
also
involves a trade-off between insertion loss due to each isolator and the total
isolation
value required.
1 o To further illustrate this aspect of the invention, FIG. 3 shows
illustrative
graphical results 300 of the total isolation that is achievable against the
number of
circulators disbursed in the power combiner arrangement of the present
invention.
In particular, total isolation (in dB) 350 is plotted versus number of
circulators per
path 360 for a variety of dB/circulator ratios (see, ratio legend 365) as
shown in
straight line plots 310 through 340, respectively. As is immediately evident
from
illustrative graphical results 300, the power combiner arrangement of the
present
invention achieves significantly higher isolation between power sources than
conventional high power combiners.
FIG. 4 shows illustrative power combiner 400 configured in accordance with
2o a further embodiment of the invention. More particularly, power combiner
400
includes hybrid coupler 405 having four input ports, ports 410-425,
respectively.
Hybrid couplers, as discussed previously, are well-known devices that couple
electromagnetic energy from an input source to multiple output ports in a
prescribed
manner. Thus, hybrid coupler 405 is used effectively with power source 430 and
power source 435 to transfer electromagnetic energy using combiner 400. That
is,
hybrid coupler 405 is used to effectively combine the two signals from power
sources 430 and 435, respectively, for output through port 410 to antenna 465.
However, we have realized that the performance of hybrid coupler 405 in a high
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power application can be significantly improved by using a series of
circulators in
conjunction with the coupler.
More particularly, in accordance with this embodiment of the invention,
circulators are employed to improve the impedance matches at the input ports
410-
425. In particular, FIG. 4 illustratively shows a series of circulators N, to
N~, see,
e.g., circulator 450 through circulator 455, respectively, displaced between
power
source 430 and port 425 of hybrid coupler 405. In accordance with the
illustrative
embodiment of FIG. 4, circulators 440-460, are each a three port junction
circulator
device formed by a symmetrical "Y" junction transmission line coupled to a
1o magnetically-biased ferrite material. As described above, when one of the
three
ports of the circulator (see, e.g., circulator 440 having ports 401, 402, and
403,
respectively) is terminated with a matched load, the circulator becomes an
isolator
which will isolate the incident and reflected signals at the remaining two
ports.
Further, as discussed above, the combination of the ferrite material, magnetic
bias
t 5 and transmission line realization determines the actual power handling
capability of
the circulator. That is, when one of the three ports of the circulator is
terminated
with a matched load, the circulator becomes an isolator which will isolate the
incident and reflected signals at the remaining two ports. Thus, in accordance
with
this further embodiment of the invention, power combiner 400 includes matched
2o loads 475-495, with each respective load being matched to a particular
circulator.
For example, circulator 450 is matched with matched load 475, and circulator
445 is
matched with matched load 490.
As above, the present embodiment also includes circulator 460 inserted
between antenna 465 and port 410 of hybrid coupler 405 to ensure that power
25 combiner 400 is matched with a sufficient impedance value. That is, the use
of
circulator 460 between the final output, i.e., port 410, of hybrid coupler 405
and
antenna 465 provides a robust interface to antenna 465 and minimizes RF power
reflected from antenna 465 from being dissipated in power combiner 400 and/or
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power sources 430 and 435, respectively. Further, leakages at port 420 are
dissipated, in a well-known manner, in balancing load 470.
As discussed above in the various embodiments, the present invention is
directed to a high power combiner arrangement with improved isolation between
input ports for high power applications. As such, our high power combiner is
used
effectively in any number of high power applications such as (i) combining two
or
more signals at the same or different frequencies for transmission by a common
antenna; (ii) combining, in a variety of manners, analog signals and/or
digital
signals for common antenna transmission, e.g., digital television and/or
digital audio
1 o broadcast applications; and (iii) combining outputs of multiple power
amplifiers, to
name just a few.
The foregoing merely illustrates the principles of the present invention.
Therefore, the invention in its broader aspects is not limited to the specific
details
shown and described herein. Those skilled in the art will be able to devise
numerous arrangements which, although not explicitly shown or described
herein,
embody those principles and are within their spirit and scope.
